An internal combustion engine mounted in a vehicle, such as an automobile, has an exhaust gas purifying catalyst provided in an exhaust passage so that NOx, HC, and CO of exhaust gas flowing through the exhaust passage are purified by the catalyst. To effectively purify these three components in the exhaust gas, the catalyst is equipped with an oxygen storage function, and stoichiometric air-fuel ratio control is executed to control an air-fuel ratio of air fuel mixture in the combustion chamber of the internal combustion engine to a stoichiometric air fuel ratio.
The oxygen storage function of a catalyst refers to a function that enables oxygen in the exhaust gas to be stored in the catalyst and the oxygen stored in the catalyst to be desorbed from the catalyst and released into the exhaust gas, in accordance with the oxygen concentration in exhaust gas passing through the catalyst. More specifically, when the oxygen concentration in the exhaust gas is higher than the value at the time of combustion of the air fuel mixture in the combustion chamber under the condition that the air-fuel ratio of the air fuel mixture is set to the stoichiometric air-fuel ratio, in other words, when the air fuel mixture in the combustion chamber is combusted at a leaner air-fuel ratio than the stoichiometric air-fuel ratio, oxygen in the exhaust gas passing through the catalyst is stored in the catalyst by the oxygen storage function of the catalyst. On the other hand, when the oxygen concentration in the exhaust gas is lower than the value at the time of combustion of the air fuel mixture in the combustion chamber under the condition that the air-fuel ratio of the air fuel mixture is set to the stoichiometric air-fuel ratio, in other words, when the air fuel mixture in the combustion chamber is combusted at a richer air-fuel ratio than the stoichiometric air-fuel ratio, the oxygen stored in the catalyst is desorbed therefrom and released into the exhaust gas by the oxygen storage function of the catalyst.
The stoichiometric air-fuel ratio control regulates a fuel injection amount of the internal combustion engine in accordance with the oxygen concentration in the exhaust gas, so that the oxygen concentration in the exhaust gas becomes equal to the value at the time of combustion of the air fuel mixture in the combustion chamber under the condition that the air-fuel ratio of the air fuel mixture is set to the stoichiometric air-fuel ratio. As disclosed in Patent Document 1, the stoichiometric air-fuel ratio control uses a catalyst upstream sensor and a catalyst downstream sensor provided on upstream and downstream sides of a catalyst in an exhaust passage, respectively. The catalyst upstream sensor is provided on the upstream side of the catalyst in the exhaust passage to output signals on the basis of the oxygen concentration in the exhaust gas. The catalyst downstream sensor is provided on the downstream side of the catalyst in the exhaust passage to output signals on the basis of the oxygen concentration in the exhaust gas.
According to the stoichiometric air-fuel ratio control using the catalyst upstream sensor and catalyst downstream sensor, the fuel injection amount of the internal combustion engine is regulated on the basis of the signal output from the catalyst upstream sensor, such that the oxygen concentration in the exhaust gas becomes equal to the value at the time of combustion of the air fuel mixture in the combustion chamber under the condition that the air-fuel ratio of the air fuel mixture is set to the stoichiometric air-fuel ratio. Accordingly, the air-fuel ratio of the air fuel mixture in the combustion chamber of the internal combustion engine, while fluctuating in a rich-lean-rich-lean sequence, is controlled to finally converge to the stoichiometric air-fuel ratio. However, when the fuel injection amount is regulated on the basis of the signal output from the catalyst upstream sensor alone, there is still a risk that a fluctuation center when the air-fuel ratio of the internal combustion engine fluctuates in a rich-lean-rich-lean sequence to converge to the stoichiometric air-fuel ratio as described above deviates from the stoichiometric air-fuel ratio due to, for example, manufacturing variability of the sensor. To correct such a deviation, the fuel injection amount of the internal combustion engine regulated according to the signal output from the catalyst upstream sensor is further regulated according to the signal output from the catalyst downstream sensor to ensure that the rich-lean-rich-lean fluctuation of the air-fuel ratio in the internal combustion engine is centered on the stoichiometric air-fuel ratio.
Because of the oxygen storage function provided in the catalyst and the stoichiometric air-fuel ratio control, three components in the exhaust gas, NOx, HC, and CO can be effectively purified. More specifically, when the air-fuel ratio of the air fuel mixture in the combustion chamber fluctuates to become lean during the stoichiometric air-fuel ratio control, the oxygen concentration in the exhaust gas passing through the catalyst is higher than the value at the time of combustion of the air fuel mixture in the combustion chamber under the condition that the air-fuel ratio of the air fuel mixture is set to the stoichiometric air-fuel ratio. Therefore, oxygen in the exhaust gas passing through the catalyst is stored in the catalyst to reduce NOx in the exhaust gas. On the other hand, when the air-fuel ratio of the air fuel mixture in the combustion chamber fluctuates to become rich during the stoichiometric air-fuel ratio control, the oxygen concentration in the exhaust gas is lower than the value at the time of combustion of the air fuel mixture in the combustion chamber under the condition that the air-fuel ratio of the air fuel mixture is set to the stoichiometric air-fuel ratio. Therefore, the oxygen stored in the catalyst is desorbed therefrom to oxidize HC and CO in the exhaust gas. Thus, while the air-fuel ratio of the air fuel mixture in the combustion chamber is fluctuating from rich to lean or from lean to rich before converging to the stoichiometric air-fuel ratio during the stoichiometric air-fuel ratio control, the three components in the exhaust gas, NOx, HC, and CO, are effectively purified.
The oxygen storage function of a catalyst increasingly declines as the catalyst is degrading with time. Therefore, there has been proposed that a maximum value of the amount of oxygen to be stored in the catalyst (hereinafter, simply referred to as oxygen storage amount) is calculated to determine whether the catalyst has degraded. The following steps are carried out to determine whether the catalyst has degraded.
When a rich-to-lean or lean-to-rich change is forced to occur in the air-fuel ratio of the air fuel mixture combusted in the combustion chamber of the internal combustion engine as illustrated in FIG. 8(a) (point in time ta), a corresponding change occurs in the signal of the catalyst upstream sensor as illustrated in FIG. 8(b) (tb). During a period from the occurrence of the corresponding change in the signal of the catalyst upstream sensor to the occurrence of a change corresponding to the change of the air-fuel ratio in the signal of the catalyst downstream sensor (tb-td), the amount of oxygen stored in the catalyst or the amount of oxygen desorbed from the catalyst is calculated. It is possible to determine that the change corresponding to the change of the air-fuel ratio has occurred in the signal of the catalyst downstream sensor when the signal reaches a determination value H, which is used to determine such a change, as illustrated with a solid line in FIG. 8(d).
When the rich-to-lean change is forced to occur in the air-fuel ratio, oxygen is stored in the catalyst during the period (tb-td). Then, the amount of oxygen stored in the catalyst during the period is calculated, and the oxygen amount calculated is then used as the oxygen storage amount of the catalyst. The oxygen storage amount thus calculated changes as illustrated with a solid line during the period (tb-td) in FIG. 8(c). When the lean-to-rich change is forced to occur in the air-fuel ratio, on the other hand, oxygen is desorbed from the catalyst during the period (tb-td). Then, the amount of oxygen desorbed from the catalyst during the period is calculated, and the oxygen amount calculated is then used as the oxygen storage amount of the catalyst. The oxygen storage amount thus calculated also changes as illustrated with a solid line during the period (tb-td) in FIG. 8(c).
To determine whether the catalyst has degraded, the oxygen storage amount obtained at the end of the period (tb-td) is compared to a threshold value for degradation assessment. When the oxygen storage amount is smaller than the threshold value, it is possible to determine that degradation of the oxygen storage function due to degradation of the catalyst has occurred, therefore, it is determined that the catalyst has degraded. When the oxygen storage amount is equal to or larger than the threshold value, on the other hand, it is possible to determine that degradation of the oxygen storage function due to degradation of the catalyst has not occurred, therefore, it is determined that the catalyst is not yet degraded (normal).